A conventional SAW device is generally manufactured by the following method. As a piezoelectric substrate, for example, a lithium tantalate (LiTaO3) single crystal substrate is used. A metal film of aluminum (Al) or the like is formed on the piezoelectric substrate, and an inter digital transducer electrode (hereafter called IDT electrode) and a grating reflector electrode are formed by photolithography and an etching process. In this way, after forming several SAW elements on the piezoelectric substrate, the substrate is diced into individual pieces. A SAW device is manufactured by mounting individual SAW elements on a ceramic package or the like. Here, the connection of the SAW element and the package pad is often made by wire bonding or flip chip bonding. In order to improve the reliability of the connection, a reinforcing electrode is further formed as needed on the pad electrode of the SAW element. Also, in the case of flip chip bonding, it is necessary to further form a bump on the reinforcing electrode.
FIG. 8 is a plan view of a conventional SAW element formed on a piezoelectric substrate. In FIG. 8, the element is shown as being formed on a piezoelectric substrate before dicing. Thus, only one SAW element is shown surrounded by a dicing line.
As piezoelectric substrate 1, a single crystal substrate such as LiTaO3 single crystal substrate or lithium nyobate (LiNbO3) single crystal substrate is used. Dicing line 2 made of Al or the like is formed on the piezoelectric substrate 1, and the SAW element is formed inside the dicing line 2. In this example, the SAW element is composed of three IDT electrodes 31, 32, 33, and two grating reflector electrodes 41, 42 disposed on either side thereof. At the input side of the outside IDT electrodes 31, 33, a ground terminal 5 and pad electrode 61 connected to the ground terminal 5 are positioned. Also, at the input side of the middle IDT electrode 32, input terminal 8 and pad electrode 62 connected to the input terminal 8 are positioned. The pad electrode 62 is surrounded by the ground terminal 5 and the pad electrode 61. Further, pad electrodes 61, 63, 64, 65, 66, 67 (not pad electrode 62) are provided with short-circuit electrode 7 connected to the dicing line 2. Here, bump 21 is formed on each of the pad electrodes 61, 62, 63, 64, 65, 66, 67.
By using such a configuration, the potential of electric charge generated on the piezoelectric substrate 1 during the manufacturing process can be made uniform by the electrodes connected to the dicing line 2, and it becomes possible to prevent a discharge from occurring between the IDT electrodes 31, 32, 33. In the case of a SAW element, dicing on the dicing line 2 causes the dicing line 2 to disappear, and the IDT electrodes 31, 32, 33 can be electrically freed.
In the SAW element manufacturing process, heat is often applied to the piezoelectric substrate during IDT electrode and grating reflector electrode or reinforcing electrode forming, as well as photolithography, etching or the like. Consequently, an electric charge generated on the piezoelectric substrate due to the pyroelectric effect of the piezoelectric substrate is accumulated. When the accumulated charge exceeds a certain level, discharge takes place between the electrodes, such as the IDT electrodes, causing the electrode pattern to be damaged and deteriorated in characteristics.
However, in the conventional configuration shown in FIG. 8, since the input terminal 8 and the pad electrode 62 are surrounded by the ground terminal 5 and the pad terminal 61, it is not possible to provide a connection electrode between pad electrode 62 and the dicing line 2. Accordingly, input terminal 8 and pad electrode 62 are kept electrically free, and discharge takes place due to the electric charge generated at IDT electrode 32, causing the electrode pattern to be damaged. An IDT electrode with a pad electrode that cannot be connected to a dicing line often exists not only in a SAW element configuration as shown in FIG. 8, but also in other element configurations.
As a means to solve this problem, a method disclosed in Japanese Laid-open Patent H3-293808 is known. In that method, the IDT electrode to be freed (as shown in FIG. 8) is configured so that the outermost electrode finger of the IDT electrode is connected to the electrode finger of an adjacent IDT electrode. Since the pad electrode connected to the adjacent IDT electrode is connected to the dicing line by a connecting line, there is no IDT electrode to be electrically freed as shown in FIG. 8. As a result, discharge can be prevented from occurring between IDT electrodes. However, in this method, even if a SAW element is employed, the IDT electrode to be freed has to be configured so that the electrode finger thereof is kept electrically connected to the electrode finger of an adjacent IDT electrode. Therefore, it becomes difficult to design so as to achieve the intended filter function for a SAW device. Further, since the IDT electrode finger is not wide enough and the impedance is great, the potential cannot be made sufficiently uniform, sometimes causing discharge to take place when a big electric charge is generated.